1. Field of the Invention
The present invention relates to a droplet ejecting apparatus such as an ink-jet printer.
2. Discussion of Related Art
There has been conventionally known, as one example of a droplet ejecting apparatus, an ink-jet printer having: an ink-jet head which includes a cavity unit in which a plurality of pressure chambers are regularly formed and a piezoelectric actuator bonded to the cavity unit for permitting ink in each pressure chamber to be selectively ejected; and a voltage application device configured to apply a voltage to the piezoelectric actuator. As such a piezoelectric actuator, there are known one that utilizes a vertical effect actuator of a stacked or laminated type and one that utilizes a unimorph actuator.
In the ink-jet head of the ink-jet printer described above, there is a demand for increasing the density of the pressure chambers to ensure a high image quality and a high quality of recording by increasing the number of nozzles in the ink-jet head. Where the pressure chambers are arranged at a high density, however, the distance between adjacent pressure chambers is reduced, so that there is caused a problem of so-called crosstalk, during driving of the actuator, in which driving of one pressure chamber influences driving of another pressure chamber that is located adjacent to the one pressure chamber.
In the light of the above, the assignee of the present application proposed a droplet ejecting apparatus in which the crosstalk can be suppressed without increasing the number of individual electrodes, namely, without increasing the number of signal lines, even when the pressure chambers are formed at a high density. The proposed droplet ejecting apparatus includes: (a) a droplet ejecting head including a cavity unit in which a plurality of pressure chambers are formed regularly and a piezoelectric actuator joined to the cavity unit for permitting a liquid in each pressure chamber to be selectively ejected; and (b) a voltage application device for applying a voltage to the piezoelectric actuator. The piezoelectric actuator includes: (i) first active portions each corresponding to a central portion of a corresponding one of the pressure chambers; (ii) second active portions each corresponding to an outer peripheral portion of the corresponding one of the pressure chambers that is located more outside than the central portion; (iii) individual electrodes each extending over both of a first region corresponding to one of the first active portions and a second region corresponding to the second active portion provided for one pressure chamber; and (iv) a first constant potential electrode disposed in the first region and a second constant potential electrode disposed in the second region.
A further study revealed the following. Where the first and second constant potential electrodes overlap each other, as seen in a superposition direction in which the cavity unit and the piezoelectric actuator are superposed, at portions of the actuator not corresponding to the pressure chambers, foreign substances tend to get caught to thereby cause cracks, and a short circuit accordingly occurs between a power source and the ground, resulting in a decrease of the withstand pressure. Further, the actuator needs to bear a large stress because the actuator suffers from a stress due to deformation of piezoelectric layers thereof. In these instances, there is a risk of breakage of the actuator. In the light of the above, each of the first and second constant potential electrodes is formed to have a comb-like shape, so as to avoid overlapping each other. That is, each of the first and second constant potential electrodes has the comb-like shape so as not to overlap each other, as seen in the superposition direction, at the portions where the foreign substances may get caught.
In the thus constructed droplet ejecting apparatus, each individual electrode needs to have a connection portion (a lead portion) through which the individual electrode is connected to a signal line (a wire). The connection portion is formed at the portions except for portions corresponding to the pressure chambers. Accordingly, the connection portion needs to be provided so as to overlap the first constant potential electrode or the second constant potential electrode each as an internal electrode, as seen in the superposition direction. The connection portion is provided with a bump formed of silver (Ag) for easy connection with a connection terminal of a flexible wiring board through which a drive signal is inputted. In the meantime, the first and second constant potential electrodes each as the internal electrode are formed of a mixture of silver (Ag) and Palladium (Pd). In general, silver (Ag) tends to suffer from migration. However, on the basis of the observation that there are no concerns of migration as long as the potential of the internal electrode that overlaps the connection portion is kept higher than the potential of the individual electrode, the connection portion was conventionally formed so as to overlap, as seen in the superposition direction indicated by “Z” (FIG. 8) in which the cavity unit and the piezoelectric actuator are superposed on each other, the first constant potential electrode to which is given a potential higher than or equal to the potential of the individual electrode.
More specifically, the piezoelectric actuator was conventionally structured as shown in FIGS. 7A, 7B, and 8. In the actuator generally indicated at 112, individual electrodes 121 are formed as a first layer on the upper surface of a piezoelectric-material layer 112a of the piezoelectric actuator 112 so as to respectively correspond to first active portions S11 for respective pressure chambers 114Aa, as seen in the superposition direction Z. First constant potential electrodes 122 are formed as a second layer on the lower surface of the piezoelectric-material layer 112a. Each first constant potential electrode 122 has a comb-like shape constituted by first branch portions 122A corresponding to the respective first active portions S11 and a first trunk portion (i.e., connecting portion) 122B to which the first branch portions 122A are connected and which extends in a direction X in which each nozzle row extends (hereinafter referred to as “the nozzle-row direction X” where appropriate). Second constant potential electrodes 123 are formed as a third layer on the lower surface of the piezoelectric-material layer 112b. Each second constant potential electrode 123 has a comb-like shape constituted by second branch portions 123A corresponding to the respective second active portions S12 and a second trunk portion (i.e., connecting portion) 123B to which the second branch portions 123A are connected and which extends in the nozzle-row direction X. The first trunk portion 122B of each of the first constant potential electrodes 122 and the second trunk portion 123B of each of the second constant potential electrodes 123 are arranged alternately in a direction Y orthogonal to the nozzle-row direction X. Connection portions 121a of the respective individual electrodes 121 that are connected to respective connection terminals of a flexible wiring board are provided so as to overlap the first trunk portions 122B of the respective first constant potential electrodes 122 as seen in the superposition direction Z. The cavity unit 111 is constituted by: a stacked body 114 in which a nozzle plate (not shown) is disposed at its underside; and a top plate 115 bonded to the upside of the stacked body 114. It is noted that arrows in FIG. 8 indicate a polarization direction.